EF context not responding in docker
I believe the issue you're running into is documented here: https://github.com/dotnet/SqlClient/issues/222. As you said, something did change with .NET Core 3 because those Docker images are based on Debian Buster by default. Buster is configured to use 1.2 as the minimum TLS protocol, a change from the previous version (see https://www.debian.org/releases/stable/amd64/release-notes/ch-information.en.html#openssl-defaults).
This can be temporarily fixed by adding the following to the Dockerfile:
RUN sed -i 's/MinProtocol = TLSv1.2/MinProtocol = TLSv1/g' /etc/ssl/openssl.cnfRUN sed -i 's/MinProtocol = TLSv1.2/MinProtocol = TLSv1/g' /usr/lib/ssl/openssl.cnfThis is not a great solution because it is essentially downgrading the version of TLS. The better long-term solution is to enable TLS 1.2 on the SQL Server (see https://support.microsoft.com/en-us/help/3135244/tls-1-2-support-for-microsoft-sql-server).